Pyrotechnic active mass for producing an aerosol highly emissive in the infrared spectrum and inpenetrable in the visible spectrum

ABSTRACT

The invention relates to a pyrotechnic active mass which is impenetrable in the visible spectrum, highly emissive in the infrared spectrum and used for camouflage and decoy purposes. As principal ingredients said mass contains red phosphorus and an alkali metal nitrate or mixture of alkali metal nitrates and as secondary ingredients at least one transition metal or a metal-rich compound or alloy thereof, at least one metalloid and a binder.

The subject of the present invention is a human and ecotoxicologicallycompatible pyrotechnic active mass consisting of red phosphorus, ametallic fuel from the group of the transition metals, preferablytitanium, zirconium or iron, a moderator of the group of the metalloidsboron end silicon, an oxidation agent of the group of the alkali metalnitrates, preferably caesium nitrate and potassium nitrate which issuitable for the production of an aerosol highly emissive in theinfrared (3-5, 8-14 μm) and impenetrable in the visual spectrum.

Pyrotechnically produced aerosols are today pre-ponderantly used in themilitary field for camouflaging, decoying, screening, simulating andmarking.

Whereas for the cases of use marking and simulating there are preferablyused coloured aerosols based on organic azo dyestuffs (white, orange,red, violet, green, blue) which only absorb in the visible range of thespectrum, for camouflaging, decoying and screening one preferably usesaerosols which also interupt in the infrared range of theelectromagnetic spectrum, especially in the range of the atmospherictransmission windows at 0.3-1.5; 1.6-1.8; 2.0-2.5; 3.0-5.0 and 8.0-14μm, by various mechanisms. To these mechanisms count the scattering,absorption and emission of radiation.

Scattering and absorption of radiation are described by the Lambert-BeerLaw

l=l _(o)exp^(−αcl)  (1)

whereby l describes the radiation intensity weakened by the reciprocalaction, l_(o) represents the initial intensity, c corresponds to theconcentration of the aerosol per volume unit, l is the path lengththrough the aerosol cloud of assumed isotropic density, α is thewavelength-dependent mass extinction coefficient of the aerosolparticles which, in the case of a given material, is made up as sum ofthe scattering and absorption coefficients:

α(λ)=α_(scat)(λ)+α_(abs)(λ)  (2)

Whereas the scattering action preponderantly depends on the particlemorphology and size of the particles, the absorption is only determinedby the chemical composition of the particles. Only the index ofrefraction m of an aerosol, which is determined not only by the physicalbut also the chemical properties, influences not only the scattering butalso the absorption behaviour.

In order that aerosols can scatter radiation, according to Rayleigh theparticle diameter, in the case of assumed spherical morphology of theparticles, and the wavelength of the radiation to be scattered must beidentical. This means that for an optimum scattering of radiation in themicrometer range, particles with particle diameters of 0.3-14 μm must bepresent.

Such particles can be produced in established way by the followingprocesses:

a) combustion of oxygen-deficient, carbon-rich pyrotechnic batches.Then, in the case of the burning, on the basis of the poor oxygenbalance, there results much carbon black with particle diameters in therelevant size range (DD 301 646 A7, DE 3326884 C2).

b) explosive dissimination of pre-produced particles, preferably brassdust in the suitable size range.

The aerosols described under a) and b) contribute to the absorption ofinfrared radiation due to their chemical composition. Not only carbonblack but also brass dust are electrically conductive and are,therefore, suitable for the decoupling of infrared radiation.

The disadvantages of the above-described methods for the production ofinfrared radiation-screening aerosol clouds consist in a) in thecontamination of the carbon black particles produced with in partcancerogenic polyaromatic hydrocarbons (PAH) and, in the case ofenergetic halogen-containing components in such pyrotechnic batches, inthe contamination of the carbon black particles with polyhalogenatedoxyarenes, such as e.g. polyhalodibenzo-furans andpolyhalodibenzodioxines or also polyhalogenated biphenylene,

In the case of the explosive dispersion of preprepared particles, italways results in so-called bird nesting. By this one understands thehole brought about by the explosive process in the aerosol cloud withvery low particle density. At this place of the cloud, the line of sight(LOS) is Dot blocked. Furthermore, the brass dust sinks very quickly tothe ground so that only unsatisfactory covering times are achieved. Thetoxic effects of brass dust on humans and the environment are also veryconsiderable so that a large-scale use must be dispensed with especiallyalso for exercise purposes.

In DE 40 30 430, an active mass is described which is produced by acoordinated amount ratio of magnesium powder, a fluoridised organicpolymer, chloroparaffin and an aromatic compound, especially anthraceneor phthalic acid anhydride which react to polyaromatics which asvoluminous agglomerates with fibrous structure, have diameters in therange of 1-20 μm which are suitable for the IR radiation scattering andabsorption and, nevertheless, because of the great specific surface,float in the air. In order to suppress the formation of finely-dividedcarbon black instead of polyaromatics, a burning speed of about 15 g/secmust be maintained so that the covering action only starts relativelylate. Therefore, in this Patent it is further suggested to add thereto arapidly burning mixture of fluorine-containing polymer, magnesium powderand an organic binder which, for a short time, in the case of theburning produces a strong IR emission and thus closes the initialcovering holes.

Disadvantageous in the case of this process is that the polyaromaticsformed also still contain cancerogenic substances and the emissiveaction subsides very quickly because of the use of magnesium.

The main problem of conventional impermeable aerosols of theabove-described type consists in the ineffectiveness effectively toprotect moving warm targets (humans, vehicles, armoured platforms)against CLOS and SACLOS missiles (e.g. Milan, TOW etc.). These missilesare controlled by means of wires or glass fibres by a controller whichaims at the target via a heat image device (8-14 μm). After targetpick-up has taken place, a controller can estimate the approximateposition from the last observed movement and, through the transmissionholes typically found in aerosol clouds, further follow the emissivetarget and direct the missile into the target.

It was, therefore, the task of the present invention to develop acamouflage smoke screen which, besides the impenetrability in thevisible range, also makes possible a long-lasting covering in the IRrange.

The solution of this task is achieved by the features of the main claimand promoted by the subsidiary claims.

The smoke screens according to the invention contain, as maincomponents, red phosphorus, an alkali metal nitrate, for example lithiumnitrate, sodium nitrate, potassium nitrate, rubidium nitrate and caesiumnitrate or a mixture thereof, as well as, as subsidiary components, ametallic fuel from the group of the transition metals, such as forexample titanium, zirconium or iron, or a metal-rich alloy or compoundof these elements, such as for example TiH, Zr/Ni, Zr/Fe or ZrSi₂, atleast one metalloid, such as for example boron or silicon or anelectron-donating compound of these elements, as well as a polymericorganic binder.

That red phosphorus serves as carrier of the transmission-dampeningaction in the visible range was long known but, on the other hand, theknowledge is new that red phosphorus, under certain circumstances, alsosets as carrier of the emissive action in the infrared range. The redphosphorus is, in the case of the reaction of the energetic componentsnitrate/metal/metalloid, substantially evaporated (equation s) and burnsin the presence of atmospheric oxygen according to equation (4) to givephosphorus pentoxide.

P_((red))+heat of combustion→P_(4(g))  (3)

P_(4(g))+5O₂→2P₂O₅+heat  (4)

Phosphorus pentoxide reacts with atmospheric moisture according toequation 5 to give phosphoric acid

P₂O₅+3H₂O→2H₃PO₄+heat  (5)

The use according to the invention of alkali metal nitrates as oxidationagent gives, in the case of combustion, alkali metal oxides which, inthe presence of atmospheric moisture, react off according to equation 6to give the hydroxides.

M₂O_((s))=H₂O→2MOH_((aq))+heat  (6)

These arosol droplets give with the phosphoric acid droplets, in astrongly exothermal reaction, the corresponding dihydrogen phosphates.

MOH+H₃PO_(4(aq))→2MH₂PO₄+H₂O+heat  (7)

The hydrating of the dihydrogen phosphates is also an exothermalreaction and again gives heat.

MH₂PO₄+n H₂O→MH₂PO₄.(H₂O)_(n)+heat  (8)

The aerosol droplets formed possess a size of 0.01-2 μm and thereby ahigh adsorption and dispersion coefficient in the visible and short-waveinfrared range of 0.3-1.9 μm and low damping values in the middle andlong wave infrared of 2-14 μm. Undamaged thereby, the heat formed by thereactions 4-6 and especially in the steps 7 and 8 provides for a strongemission of the aerosol droplets in the medium and long waved infraredand thus compensates the low scattering and absorption coefficients inthis spectral range. In contradistinction to the known strong emissionof magnesium-containing active masses which occurs directly in the caseof the combustion and thereafter subsides quickly, the heat developmentaccording to the invention occurs partly by chemical processes whichfirst become possible by the delayed formation taking place of theaerosol droplets so that this emissive action is maintained for 50-200sec., i.e. the time necessary for a camouflaging.

Due to the use according to the invention of transition metals theoxides of which have high heats of formation, such as for examplezirconium and titanium, as well as of metalloids, such as boron and/orsilicon, very high combustion temperatures are achieved, therefore theaerosol particles maintain a high thermal energy which increases theemission in the longwaved IR.

Furthermore, the use according to the invention of the transition metalsand their alloys or metal-rich compounds suppresses the formation ofphosphane formers. The metal phosphides (e.g. zirconium phosphide ortitanium phosphide) formed due to the oxygen underbalancing possess anon-ionic character, for which reason no hydrolysis or acidolysis withthe liberation of phosphanes takes place with atmospheric moisture oracidic rain.

Therefore, smoke screens produced according to the invention are humanand ecotoxocologically compatible and considerably, safer thanconventional smoke screens based on red phosphorus and light metal, suchas for example magnesium or aluminium. The self-ignition of thecombustion residues typically occurring in the case of smoke screensbased on red phosphorus is thus also no longer given.

The following Example is to explain the invention without limiting itthereto:

EXAMPLE

From 2750 g red phosphorus, 990 g potassium nitrate, 220 g silicon, 220g boron, 220 g zirconium and 990 g macroplast binder (30% solid bodies)is produced a pasty batch by stepwise addition of the components to thered phosphorus. The solvent-moist mass is sieved (7 mm mesh width) anddried for 20 minutes in a vacuum at 40° C. and 20 mbar. The 42 g ofgranulate are pressed with a moulding pressure of 20 tonnes intoring-shaped pressed bodies. of 10 mm edge height, 57 mm externaldiameter and 15 mm internal diameter. A tablet possesses 8 burning timeof about 35 seconds and in visual light produces a thick white smoke.

A radiometric measurement of the resulting aerosol at 4 m distance fromthe source discloses the following radiation strengths in the infraredrange;

band V (8-14 μm) Band II (3-5 μm) >100 W/sr > 25 s >20 W/sr > 25 s  >60W/sr > 75 s  >10 W/sr > 75 s.

FIG. 1 shows the radiation strengths of the aerosol clouds which areproduced by combustion of a pressed body of the weight 120 g producedaccording to the invention at 5 m distance from the source. With theaerosol clouds produced according to the invention, there is achieved avery good irradiation (>95%) of emissive targets, the colour temperaturereaches 300° C.

What is claimed is:
 1. A pyrotechnic active mass for camouflaging anddecoy purposes which is impenetrable in visible light and stronglyemissive in infrared light, comprising, as main components, 45% to 75%red phosphorus and 15% to 35% of alkali metal nitrate or a mixture ofalkali metal nitrates, as subsidiary components, 2% to 20% of at leastone transition metal or a metal-rich compound or an alloy thereof and atleast one metalloid, and 0.5% to 8% of a binder.
 2. The pyrotechnicactive mass of claim 1, which, upon reaction, forms aerosol droplets. 3.The pyrotechnic active mass according to claim 1, comprising 55% to 62%red phosphorus, 18% to 23% alkali metal nitrates, 10 to 18% metals ormetalloids and 5 to 7% binder.
 4. The pyrotechnic active mass accordingto claim 1, comprising 58.5% red phosphorus, 21.1% potassium nitrate,4.7% each of boron, silicon and zirconium, and 6.3% of a polychloroprenebinder.
 5. The pyrotechnic active mass of claim 2, wherein said aerosoldroplets are 0.3 to 14 μm in size.